High resolution tuning over a wide frequency range

ABSTRACT

A METHOD AND APPARATUS FOR ACCURATELY TUNING THE FREQUENCY OF A GENERATED SIGNAL BY COARSE TUNING THE OUTPUT OF A GENERATOR IN A FIRST STAGE AND PROVIDING A PLURALITY OF FURTHER TUNING STAGES EACH ARRANGED TO EFFECT A FINER TUNING THAN THE PRECEDING STAGE, EACH FURTHER STAGE BEING ARRANGED TO GENERATE A SIGNAL, TO MIX THAT SIGNAL WITH THE SIGNAL FROM THE PRECEDING STAGE TO PRODUCE AN OUTPUT WHOSE FREQUENCY IS EQUAL TO THE DIFFERENCE BETWEEN THE FREQUENCIES OF THE TWO MIXED SIGNALS, FREQUENCY DIVIDING THE DIFFERENCE SIGNAL BY MEANS OF A VARIABLE DIGITAL COUNTER WHICH IS CONTROLLED TO HAVE ANY ONE OF A PLURALITY OF FREQUENCY DIVISION RATIOS, COMPARING THE FREQUENCY OF THE COUNTER OUTPUT WITH A REFERENCE FREQUENCY DERIVED FROM A QUARTZ OSCILLATOR, PRODUCING A CONTROL SIGNAL WHICH IS PROPORTIONAL TO THE DIFFERENCE BETWEEN THE COMPARED FREQUENCIES, AND APPLYING THIS CONTROL SIGNAL TO THE GENERATOR OF THE ASSOCIATED STAGE TO BRING THE OUTPUT FREQUENCY OF THE GENERATOR TO A VALUE WHICH WILL CAUSE THE COUNTER OUTPUT FREQUENCY TO BE EQUAL TO THE REFERENCE FREQUENCY OF THE GENERATOR TO A VALUE WHICH WILL CAUSE THE THE GENERATOR OF THE LAST FURTHER STAGE.

United States Patent 11.5. Cl. 330-43 6 Claims ABSTRACT OF THE DISCLOSURE A cold cathode distributed emission reentrant type crossed field amplifier is provided which possesses electrical efiiciencies approaching that available in the reentrant type crossed field amplifiers of the prior art but which features the simplicity of the non-reentrant type crossed field amplifier. A circular configuration is used for the tube; one in which the beginning and end of the interaction region is connected with the drift space region, so as to form a complete annular passage abou the sole and cathode. The magnetic structure produces a magnetic field at the programmed level, which may be constant, in the interaction region and in addition provides a variation in the drift space. At the entrance to the drift space the magnetic intensity decreases as a function of position along the length of the drift space from the programmed level, gradually, over a predetermined length of the drift space to a second lower predetermined intensity. Along the remaining length of the drift space the field intensity increases in level until at the end of the drift space, coincident with the beginning of the interaction region, the magnetic field intensity is again at the programmed level. The tapering of the magnetic field in the drift space region is designed so that it diverts significant amounts of those electrons that pass through the interaction region and enter the drift space region. Hence, the remaining electrons which reenter the interaction region are insufficient in quantity or energy to cause self-oscillation after the RF drive signal has been removed.

This invention relates to cross field amplifiers and, more particularly, to cold cathode, distributed emission, forward wave cross field amplifiers.

Distributed emission crossed field amplifiers have heretofore been available in either reentrant or non-reentrant beam configurations for the purposes of amplifying very high frequency pulse signals generally used in laboratories or radar systems. The non-reentrant beam configuration, familiar in the linear format such as the Dematron sold by Litton Industries of San Carlos, Calif, is of a structure which includes a length of slow wave structure spaced from and confronting a length of high secondary emission material forming a cold cathode. The space between those elements is defined as the interaction region. Side by side with the cathode and slow wave structure and spaced apart by a distance substantially the same as that of the interaction region is a sole and a collector electrode. The space formed between the latter two elements is defined as the drift space. An electric field is applied between the cathode and slow wave structure and a magnetic field is applied perpendicular or crossed to the electric field. In such a linear configuration the electrons generated due to the application of an RF drive signal to the slow wave structure forms a beam, eifectively, over the surface of the cathode in the interaction region.

In such non-reentrant configuration the crossed field amplifier becomes completely self-modulating. That is, the electron beam obtained by electrons from the cold secondary emission cathode is automatically turned-on by the input RF drive signal and is extinguished a few nanoseconds after the RF drive signal ceases.

On the other hand the reentrant beam crossed field amplifier is of a circular configuration in which the interaction region and the drift space form a continuous ring like circular passage about a cylindrical shaped sole and cathode. The slow wave structure confronts a length of cathode and both are shaped as arcs of a circle. Complementary circular arcs are formed by the space confronting the sole and collector electrodes to form an annular passage.

In present reentrant type crossed field amplifier configurations a portion of the sole is removed and a control electrode is inserted therein electrically insulated from the sole. This element is connected to a source of high voltage pulses. An RF drive signal applied to the input of the slow wave structure turns-on the electron beam in the same manner as the non-reentrant tube. However, in the operation of this device part of the electron beam generated in and passing through the interaction region continues out the end of the interaction region around into the drift space and therearound back into where they reenter the interaction region. When the RF signal is extinguished or removed, however, the movement of electrons through the interaction and drift space regions continues in a self-limited, self-generating condition. In short, when the drive signal is removed the tube commences oscillation and continues to oscillate unless the control or shutoff electrode is properly energized.

Accordingly, to operate existing reentrant tube configurations a high voltage pulse generator or modulator is connected to the control electrode which is synchronized with the RF drive signal so that when the RF drive signal is removed the modulator applies a large voltage pulse to this control electrode. That voltage appears across the drift space and, under the influence of this high voltage, the beam of electrons traveling through the drift space is diverted from its path of travel tothe control electrode. Inasmuch as those electrons have been diverted and cannot then reenter the interaction region, tube oscillation stops. Thus, proper operation of present reentrant type crossed field amplifiers requires that a control electrode be included and that the electron beam be shut 01f at the termination of each RF drive pulse.

Because many electrons do not fully interact with the electromagnetic fields in the interaction region of the non-reentrant type cross field amplifier and continue into the drift space with some potential energy, they cannot be collected at cathode potential and their energy is dissipated at the collector. It is estimated that approximately 35% of the energy available in the electrons is not fully utilized in the interaction region and is isdissipated as wasted energy in the collector. Accordingly, in contrast to the reentrant format where electrons continue to circulate around and around through the interaction and drift space the non-reentrant format results in a lower electrical efliciency.

By contrast the reentrant type crossed field amplifier, although more efficient than the non-reentrant type, continues to oscillate unless the shut-off or control voltage is applied to an additional shut-off or control electrode. As explained, this requires both additional equipment and tube structure which makes the construction of a complete amplifier system more complex, heavy, and expensive.

Accordingly, it is an object of this invention to provide a crossed field amplifier having an efficiency approaching that of the reentrant type crossed field amplifier but which does not require the additional control or shut-off electrode required in reentrant type tubes.

United States Patent 3,560,868 HIGH RESOLUTION TUNING OVER A WIDE FREQUENCY RANGE Helmut Oberbeck, Backnang, Germany, assignor to Telefunken Patentverwertungsgesellschaft m.b.H., Ulm,

Danube, Germany Filed Sept. 19, 1968, Ser. No. 760,846 Claims priority, application Germany, Sept. 22, 1967, P 15 91 722.3 Int. Cl. H03b 3/04 US. Cl. 33l--2 5 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for accurately tuning the frequency of a generated signal by coarse tuning the output of a generator in a first stage and providing a plurality of further tuning stages each arranged to effect a finer tuning than the preceding stage, each further stage being arranged to generate a signal, to mix that signal with the signal from the preceding stage to produce an output Whose frequency is equal to the difference between the frequencies of the two mixed signals, frequency dividing the difference signal by means of a variable digital counter which is controlled to have any one of a plurality of frequency division ratios, comparing the frequency of the counter output with a reference frequency derived from a quartz oscillator, producing a control signal which is proportional to the difference between the compared frequencies, and applying this control signal to the generator of the associated stage to bring the output frequency of the generator to a value which will cause the counter output frequency to be equal to the reference frequency, the output from the system being taken from the generator of the last further stage.

BACKGROUND OF THE INVENTION The invention relates to frequency tuning in accordance with the principle of the digital counting method in successive stages each of which effects a finer tuning than the preceding stage. The invention is particularly concerned with an arrangement and method wherein, in each tuning stage, a frequency produced in a generator, which frequency is electronically variable and which is frequency divided in a variable digital counter, is compared, in a discriminator, with a reference frequency derived from a quartz oscillator, and the thus-produced direct voltage is employed, by way of a variable-gain amplifier, for tuning the generator in accordance with the digital counter division ratio.

There are many situations in which the need exists for producing an output signal Whose frequency can be varied over a Wide range in uniformly spaced steps. In addition, it is often desirable that the signal frequency be variable from one value to another by purely electronic means and in an extremely short period of time. It would also be highly advantageous if such a signal could be produced without the aid of turnable frequency filters and with a minimum of fixed filters.

These requirements must be imposed because, on the one hand, filters which are tunable over a large frequency range become voluminous and expensive, due to the coils required for this purpose, and, on the other hand, because the electronic tuning of these filters is extremely difficult. Moreover, a signal-to-noise ratio of at least 75 decibels for the output signal is generally required. Furthermore, the requency range pattern should be related, if possible, to only one quartz oscillator-derived frequency.

These requirements could be met either by a so-called frequency synthesis, wherein, by frequency multiplicaice tion, frequency mixing and frequency division, new frequencies are produced by combination, or by frequency analysis, wherein a freely oscillating generator is connected to a quartz oscillator output.

SUMMARY OF THE INVENTION It is a primary object of the present invention to meet the above requirements in an improved manner.

Another object of the invention is to permit an output signal to be tuned over a wide frequency range in extremely fine frequency steps in a highly accurate and rapid manner.

Yet another object of the invention is to permit an output signal to be tuned over a wide frequency range with the aid of but a single quartz oscillator.

Still another object of the invention is to produce such a signal having a high signal-to-noise ratio.

Yet a further object of the invention is to produce such a result while eliminating the need for any tunable filters and while reducing the number of fixed filters required.

These and other objects according to the invention are achieved by certain improvements in a method for tuning an output signal to any selected one of a plurality of incrementally spaced frequencies according to the digital counting technique, which tuning is performed in successive stages in each of which the frequency of a generator output is divided by a digital counter having a controlled variable frequency division ratio, the counter output frequency is compared with a reference frequency derived from a quartz oscillator to produce a control voltage proportional to the difference between the frequencies being compared, and the control voltage is applied to the generator to cause the generator output to have a frequency such that the frequencies being compared are identical. The improvements according to the invention include providing at least two such stages and. feeding the generator output from one stage to the next succeeding stage, mixing the generator output of each stage but the first with the generator output of the next preceding stage to produce a difference signal whose frequency is equal to the difference between the frequencies of the two mixed outputs, frequency dividing each such difference signal in a variable digital counter, and controlling each such counter to give it any one of a plurality of frequency division ratios which are selected for causing the frequencies to which the generator of its associated stage can be tuned to be more closely spaced than those of the preceding stage.

The invention also includes apparatus for performing the above-described method.

In accordance with a preferred embodiment of the present invention, the reference frequencies for all of the stages are derived from a single quartz oscillator, each reference frequency being derived at the output of a fixed frequency divider connected to the quartz oscillator. The desired output signal is taken from the output of the gen erator of the last stage.

In further accordance with the invention, the tuning of the first stage is accomplished by means of a fixed frequency divider connected between the generator and the digital counter, the frequency division ratio of the fixed divider determining the frequency increments between the frequencies which can be produced by the generator of the first stage, depending on the count which the counter of that stage is adjusted to produce.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block circuit diagram of a general tuning system according to the invention.

FIG. 2 is a block circuit diagram of one embodiment of a tuning circuit according to the invention.

3 DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, the generator G1 in the first separating stage A1 is arranged to produce a signal having the frequency f1. The frequency of this signal is divided by the desired amount in the succeeding coarse tuner R1 having a divider included therein, which coarse tuner can be electronically controlled to vary its division ratio, and the frequency of the tuner output is compared in a discriminator D1 with the fixed frequency of the output from a quartz oscillator Q1. The output from discriminator D1, which is proportional to the difference between the frequencies, is fed to the generator G1 for fine tuning the generator to the desired tuned frequency.

In the next separating stage A2, the generator G2 produces a signal having the frequency '2. This signal at frequency f2 is heterodyned on the signal at frequency f1 from the first stage A1 in a mixer M2 to produce a difference frequency M2. In a subsequent tuner R2, there is performed a tuning of the difference frequency Af2 which is finer than that performed by tuner R1 of stage A1. The more finely tuned difference frequency is compared, in a discriminator D2, with the fixed frequency fQ2 of the output of an associated quartz oscillator stage Q2, and the comparison voltage representing the difference between these frequencies is fed to the generator G2 of the same stage A2 for adjusting the frequency of this generator. If the output of this more finely tuned generator G2 of stage A2 is fed to further separating stages A3 to An producing outputs at frequencies f3 to fn, and, in each case, the difference frequency Af3-Afn is formed in the mixer M3 to Mn, then the variable frequency division increments can be made finer and finer from stage to stage. The most finely tuned signal fn is derived at the output of the generator Gn of the stage An, which generator is connected with the output A.

FIG. 2 shows an example of a particularly advantageous embodiment of the circuit according to the invention. Here, the generator G1 is controllable to produce a signal in the frequency range of 230-460 mHz., the generator being tunable in steps of 10 mHz., as will be explained below. The signal is fed from the generator G1, by way of a buffer amplifier TV1, to the mixer M and to a frequency divider T 1 having a fixed dividing ratio which is here selected to be 25:1. An electronically variable counter Z1 is connected thereafter, which counter can be electronically adjusted to count any integral number of cycles between 23 and 46, and hence to have any integral frequency division ratio between 23:1 and 46:1.

The frequency of the output of the counter is compared, in the discriminator D1, with a 0.4 mHz. signal produced by suitable frequency dividing, in divider T1, the 2 mHz. output of the quartz oscillator QG. The output of the discriminator D1, representing the difference in frequency between its input signals, is fed to the generator G1 via a variable-gain amplifier RV1 having a sufficient bandwidth to enable its use as a negative feedback branch, and via a subsequent low-pass filter included in the variable-gain amplifier. The count produced by counter Z1 will determine the frequency which generator G1 Will be adjusted to produce. Thus, a count of n=23 results in a frequency of 230 mHz., while a count of n:46 produces an output signal frequency of 460 mHz. With this arrangement, a sufficient accuracy of the generator G1, and also of the generator G2, with respect to the desired tuning frequency is ensured.

The signal at the frequency f1 applied to the mixer M by the generator G1 is mixed with the signal at a frequency f2 of between 240 and 480 mHz. from the generator G2, this latter signal being conducted to mixer M via a second buffer amplifier TV2, and a difference frequency signal is obtained therefrom having a frequency Af2 of between 10 and 19,875 mHz. After the signal is sent through a subsequent bandpass filter BP, it is fed to a digital cycle counter Z2 which is adjusted to effect the fine tuning of the output of generator G2, this output being tunable to various frequencies spaced from one another by intervals of 125 kHz. The adjustment of generator G2 is effected by comparing, in comparator D2, the frequency of a 125 kHz. signal derived from a frequency divider T2 connected to oscillator QG with the frequency of the output from counter Z2. The voltage resulting from the comparison is applied, as the adjusting or tuning criterion, to the generator G2, by Way of the variable-gain amplifier RV2, which also contains a bandpass filter, to tune the generator G2 to produce an output Whose frequency is such as to cause the output frequency from counter Z2 to equal 0.125 mHz. This tuning arrangement simultaneously provides a negative feed back for the generator G2, to improve its signal-to-noise ratio.

The frequency of the output produced by generator G2 is first coarsely selected to be within a selected 10 mHz. range by the frequency 11 of the output from generator G1, this being determined by the adjustment of counter Z1. For example, if counter Z1 is adjusted to produce an output pulse, or cycle, in response to every 30 cycles which it receives (rt-:30), the output from generator G1 will be controlled so that its frequency is maintained at 300 mHz. This will cause the output from generfrom Z2 to have a frequency of 0.125 mHz., this being the frequency required for discriminator D2 to produce an indication that equality exists between the two frequencies which it compares. In order for the output from mixer M to have a frequency of 12.5 mHz., the output from generator G2 must have a frequency of 312.5 mHz. Thus, the output from discriminator D2 will act on generator G2 to bring, or maintain, its output to a frequency of 312.5 mHz. If counter Z2 were adjusted to produce a count of m=l0l, then the output of generator G2 would have to be at a frequency 312.625 mHz. for the frequencies of the two inputs to discriminator D2 to be equal.

This circuit arrangement thus makes it possible to subdivide the frequency band f2=240-480 mHz. selectively, in an electronically controllable manner, with a tuning interval of 125 kHz. while obtaining a signal-tonoise ratio of 75 decibels at output A. Due to the electronic operability of the frequency selecting counters Z, remote control is made possible, and furthermore, no tunable filters are required. Besides, by an appropriate selection of the frequencies and frequency selection increments, it is possible to tie the entire frequency range to only one quartz oscillator.

All of the individual units of the arrangements shown in FIGS. 1 and 2 can be constituted by well known, commercially available components.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations.

I claim:

1. In a method for tuning an output signal to any selected one of a plurality of incrementally spaced frequencies according to the digital counting technique, which tuning is performed in successive stages in each of which the frequency of a variable frequency signal produced in that stage is divided by a digital counter having a controlled variable frequency division ratio, the counter output frequency is compared with a reference frequency derived from a quartz oscillator to produce a control voltage proportional to the difference between the frequencies being compared, and the control voltage is applied to a generator to cause the generator output to have a frequency such that the frequencies being compared are identical, the improvement comprising: providing at least two such stages and feeding the generator output from one stage to the next succeeding stage; mixing the generator output of each stage but the first with the generator output of the next preceding stage to produce a difference signal which constitutes the variable frequency signal produced in said each stage but the first and whose frequency is equal to the difference between the frequencies of the two mixed generator outputs; frequency dividing each such difference signal in the digital counter; and controlling each such counter to give it any one of a plurality of frequency division ratios which are selected for causing the frequencies to which the generator of its associated stage can be tuned to be more closely spaced than those of the preceding page.

2. A method as defined in claim 1 comprising the further step of deriving the reference frequencies for all of the stages from a single quartz oscillator.

3. A method as defined in claim 1 wherein the frequency of the generator output of the first stage is divided in a fixed frequency divider and the output of this fixed frequency divider is delivered to the digital counter of the first stage.

4. A method as defined in claim 1 wherein the tuned output signal is constituted by the generator output of the last such stage.

5. In an arrangement for tuningan output signal to any selected one of a plurality of incrementally spaced frequencies according to the digital counting technique, which arrangement includes successive stages in each of which the frequency of a variable frequency signal produced in that stage is divided by a digital counter having a controlled variable frequency division ratio, means are provided for comparing the counter output frequency with a reference frequency derived from a quartz oscillator to produce a control voltage proportional to the difference between the frequencies being compared, and the comparing means is connected to apply the control voltage to a generator of its associated stage to cause the generator output to have a frequency such that the frequencies being compared are identical, the improvement wherein there is a first stage and at least one further stage, each said further stage being connected to receive the generator output from its immediately preceding stage, and each said further stage comprising means for mixing the generator output of said further stage with the generator output of its immediately preceding stage to produce a difference signal which constitutes the variable frequency signal produced in said further stage and Whose frequency is equal to the difference between the frequencies of the two mixed generator outputs; and said digital counter is connected to the output of said mixing means for dividing the frequency of each such difference signal, said counter being controlled to have any one of a plurality of frequency division ratios which are selected for causing the frequencies to which the generator of its associated stage can be tuned to be more closely spaced than those of the immediately preceding stage.

JOHN KOMINSKI, Primary Examiner U.S. C1. X.R. 331-16, 22 

